The use of quicklime and similar alkaline materials for sanitation has a long history. The Roman army was known to spread quicklime in their latrines. In the last century, with the development of modern sewage systems, the use of lime products for flocculation of solids in raw sewage or liquid sludges has been developed. The role of alkaline materials in pH adjustment and the beneficial effect of pH in pathogen reduction is a more recent development, having occurred in only the last 60 years. The end result of research into pathogen reduction in wastewater treatment has prompted the U.S. Environmental Protection Agency to promulgate regulations specifying environmentally sound treatment criteria. More specifically, the EPA's standards for the use or disposal of sewage sludge (40 CFR 503; "Methods for Treatment and Disposal of Sewage Sludge," Feb. 19, 1993) establishes methods for achieving pathogen and vector attraction reduction using alkalinity (pH) and temperature criteria.
The aforementioned EPA regulations outline a number of chemical processes allowed to treat sludge to render it suitable for beneficial use in agriculture and similar application. The regulations establish two classes of sludge treatment products: Class A and Class B. A Class A (former PFRP or Process to Further Reduce Pathogens) product can be achieved by heating the sludge to no less than 70.degree. C. for no less than 30 min. (pasteurization) while simultaneously raising the product pH to no less than 12 and keeping this pH level for a predetermined amount of time (pH of 12 for 2 hours followed by a pH of 11.5 for 22 hours). Adding quicklime to dewatered sludge can achieve the aforementioned pH, temperature and time conditions.
A majority of research in this field involves the pathogenic stabilization of sludge through pH adjustment and heat generated from the exothermic reaction of quicklime with water. U.S. Pat. No. 4,270,279 issued to Roediger and U.S. Pat. No. 4,306,978 issued to Wurtz disclose methods utilizing such research. U.S. Pat. No. 4,270,279 discloses the gentle handling of partially dewatered sludge cake and dusting only the surfaces of sludge particles thus resulting in a granular product; however, this process can only be carried out using partially dewatered sludge cake in sheet form prepared by belt filter presses.
U.S. Pat. No. 4,306,978 discloses a process which utilizes dewatered sludge cake from any source, however a high dose of quicklime is necessary in carrying out the process. The thorough mixing of the quicklime in this process, as compared to only surface dusting in the Roediger process, is a major reason for the increased quicklime usage in this process. In actual tests using the same type sludge blender as described in U.S. Pat. No. 4,306,978, that being the blender described in U.S. Pat. 3,941,357, it was found that 25% more quicklime was required to produce a granular product than is needed to meet EPA PFRP pH and temperature standards.
Enhanced granularization or pelletization of sludges has been taught in a number of disclosures such as U.S. Pat. No. 3,963,471 issued to Hampton, U.S. Pat. No. 4,956,926 issued to Glorioso, and U.S. Pat. No. 5,069,801 issued to Girovich. Each of these patents teach the recycling of a portion of the pelletized end product to promote further drying and pelletization. The drawback to each of these systems is that all of the recycle material must be reheated because the recycled product is obtained from the end of the process and after the product has been subjected to a drying process where the material inherently cools. In doing so, valuable heat energy is lost.
Lime is the major expense in the lime treatment process. For example, in order to meet current Class A (former PFRP) requirements mandated by U.S. EPA (40 CFR, Part 503) mentioned above significant amounts of lime are to be used. Lime ratio, i.e. tons of lime per dry ton of sludge must be in the range of 0.9 to 2.0 depending upon moisture content of the incoming sludge (usually between 15% and 35% total solids) and the end product dryness required for beneficial use (usually 45% to 60% total solids).
Accordingly, there is clearly a need for a process which can meet the EPA Class A pH, temperature and time standards while minimizing the amount of quicklime necessary in carrying out the process. Moreover, there is a need for a process which efficiently and effectively achieves a virtually pathogen free end product which is unsuitable for the regrowth of undesirable organisms. Furthermore, there is a need for a process which minimizes particulate and gaseous emissions while producing a durable granular or pelletized sludge product for use as a soil amendment or fertilizer through a more energy efficient granulation or pelletization method.